Unlike bulk materials, nanoparticles may adjust their physical characteristics (e.g., energy bandgap and melting point) by changing their size. For example, a semiconductor nanocrystal (also known as a quantum dot) is a semiconductor material having a crystalline structure of a size of several nanometers. The semiconductor nanocrystal has a very small size so that it has a large surface area per unit volume and may exhibit a quantum confinement effect. Therefore, the semiconductor nanocrystal has different physicochemical characteristics from the bulk material. A quantum dot may absorb light from an excitation source to be in an excited state, and may emit energy corresponding to its energy bandgap. The energy bandgap of the quantum dot may be controlled by varying its size and/or the composition thereof. In addition, the quantum dot may emit light with high color purity. Therefore, various applications of the semiconductor nanocrystal in a display element, an energy device, a bio-light emitting element, or the like have been researched.
A semiconductor nanocrystal (i.e., a quantum dot) may be synthesized by a vapor deposition method such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), or by a wet chemical method of adding a precursor to an organic solvent to grow crystals. In the wet chemical method, an organic material such as a dispersant coordinates a surface of the semiconductor crystal during the crystal growth to control the crystal growth. Therefore, the nanocrystals produced by the wet chemical method usually have a more uniform size and shape than those produced by the vapor deposition method.
Ga—Se semiconductor nanocrystals draw much attention as one of the base compositions for a copper-indium-gallium-selenide (CIGS) nanocrystal that is commonly used for photovoltaic cells, but little has been known about how to make them. Moreover, it is known that the Ga—Se semiconductor nanocrystals are very difficult to synthesize via the wet chemical method. Thus, an urgent need to develop technologies to synthesize the Ga-se nanocrystal via the wet chemical method remains.